This invention relates to a device for measuring and indicating flow around a bend. The present invention consists of a cavity connecting to a curved segment of a flow conducting pipe or conduit and employs the dynamics of the curved flow to generate a secondary flow vortex in the cavity and cause an indicating rotor to rotate therein. The flow through the bend can be visually indicated by the rotational motion of the rotor or the flow rate can be determined by the speed of the rotor as the rotor speed increases monotonically with the flow rate.
Measurement of fluid flow in pipes and conduits is one of the common tasks in industry and in laboratories. There are many measurement methods currently in use for different fluids, flow rate ranges, and accuracy requirements. While they vary in measurement techniques, they all measure the main flow directly. As a consequence most common flow measurement systems interfere with the flow field being measured and cause undesirable effects, in particular, high pressure drop. A common type of flow measuring instrument, turbine flow meters, consists of a rotor with turbine blades which essentially span over the main flow path. The fluid flow is guided through the turbine blades and turns the turbine and the flow rate is deduced by monitoring the rotating speed of the turbine. In this measurement process the main flow interacts with the turbine blades and some of the flow energy is converted to rotational motion and turbulence which subsequently dissipated as heat.
In-line sight flow indicator is another useful flow monitoring device which offers a simple, inexpensive means of identifying flow and direction, as well as the color and clarity of the fluid. Sight flow indicators are used to provide a reliable verification of flow in filter, lubrication, and cooling lines and to provide a positive and reliable backup for meters, switches, process indicators, and other control devices. Since sight flow indicators provide a visualization of flow they are readily interpreted by the observer. A number of different sight flow indicators have been developed to serve specific needs. The rotary sight flow indicator has been found to be the best way to show flow in opaque fluids because of the high visibility of the rotor. The rotary sight flow indicator is also preferred in certain applications because the motion of the rotor is visible from a distance.
Heretofore, rotary sight flow indicators have been designed to be used in a straight pipe section. In order to cause the rotor to turn in the conventional design, a rotary flow indicator is generally constructed with a flow diverting element at the inlet of the indicator to direct flow to one side of the the rotor. The asymmetric flow impingement on the rotor results in an unbalanced torque which causes the rotor to turn such that rotor blades upon which the flow is impinging move in the direction of the flow. This movement of the rotor blades, which can be visually observed through a view port, provides a positive indication of flow in the fluid. In a flow indicator capable of displaying flow in either direction (bi-directional), two such flow diverting elements are required, one at either end of the cavity.
The most undesirable characteristic is the high pressure drop arising from the flow diverting element which partially blocks the flow path and consequently introduces a large resistance to flow by decreasing the flow area at the inlet to the cavity. High pressure drop translates to high energy consumption and high operating costs. In fact, this high pressure drop is the main reason why rotary flow indicators are considered less appropriate than other types of flow indicators in many applications, in spite of the fact that rotary flow indicators provide a better visual flow indication. Another disadvantage of the conventional rotary flow indicators is their insensitivity to low flows. In order to enhance the low flow sensitivity of a conventional rotary sight flow indicator a strong flow deflection is required to induce rotation of the indicating rotor. Increasing the amount of flow deflection is achieved by increasing the flow diverter blockage at the inlet which results in an increased pressure drop at elevated flows. Even with the penalty of increased pressure drop resulting from the enhanced flow diversion, the conventional design still fails to provide the desired flow sensitivity at low flows. The flow deflection required to turn the indicator rotor results in a pressure drop through the indicator which is too high for many applications. In summary, the conventional rotary flow indicators are designed to observe the flow at a straight pipe section they causes far too much flow resistance. Heretofore, no rotary flow indicators have been available which indicate the flow at an elbow and all conventional rotary flow indicators consume significant amounts of flow energy.
The invention as described herein introduces a new approach for flow measurement and for sight flow indication. The working principle is based on inducing and measuring a secondary flow vortex to deduce the flow rate of the main fluid flow. This new approach involves a modification of a curved main flow path by adding a cavity joined to the main flow at the curved section in such a manner that a secondary vortex flow is generated therein. The main flow is measured by quantifying the strength of the vortex induced in the cavity. A great advantage of this new approach is that it causes only minimum interference to the main flow and as a consequence it produces little or no additional pressure drop since it is not interposed in the main flow path. In fact, introducing a properly designed cavity in a flow bend can result in more favorable local flow conditions and the overall pressure drop around the bend can be lower than that with out the cavity. Clearly, such a reduction in pressure drop can not be achieved by existing flow measurement methods.
The reason for the observed reduction has to do with the boundary layer structure in curved flow and the flow separation which occurs in this boundary layer, causing most of the irrecoverable flow losses, or pressure drop, associated with an elbow. The existence of a flow cavity redefines the boundary layer structure which can prevent flow separation which would otherwise occur by replacing the small local flow reversal loops at the wall with a single large flow reversal loop in the cavity. Flow bends, such as elbows, are extremely common in piping systems and the flow meter, in accordance with the present invention, can be installed in place of an ordinary flow bend. It is therefore realistic to expect that simultaneous with a flow measurement, a pressure reduction can be achieved.
The working principle of the present invention, which can be applied equally well to flow measuring devices as to a sight flow indicator, is drastically different from that of common turbine flow meter and rotary flow indicators. These differences can be enumerated as follows:
A. All conventional turbine flow meters and sight flow indicators are design for used in a straight pipe section. The present disclosed invention is specifically for measuring and indicating a flow around a flow bend.
B. In the conventional turbine flow meter or sight flow indicator all of the main flow is forced to pass through the cavity containing the rotor. In accordance with the present invention, the rotor is housed in a cavity that is adjacent to the main flow path and the main flow does not pass through the rotor. The rotor interacts with a secondary flow of the band flow but not the main flow.
C. Conventional turbine meters interact with all of the flow in a configuration in which the flow is basically straight and the rotor must rely on the lift of the blades to produce the rotational force. The flow meter described in accordance with the present invention is designed to measure the strength of a counter-rotating secondary vortex co-axial with the rotor with the rotational force being provided by the vortex flow.